An isotopic perspective on the correlation of surface ocean carbon dynamics and sea ice melting in Prydz Bay (Antarctica) during austral summer

The stable carbon isotope composition of particulate organic carbon (delta C-13(POC)) and naturally occurring long-lived radionuclide Ra-226 (T-1/2=1600 a) were applied to study the variations of upper ocean ( < 100 m) carbon dynamics in response to sea ice melting in Prydz Bay, East Antarctica during austral summer 2006. Surface delta C-13(POC) values ranged from -27.4 parts per thousand to -19.0 parts per thousand and generally decreased from inner bay (south of 67 5) toward the Antarctic Divergence. Surface water 226Ra activity concentration ranged from 0.92 to 2.09 Bq/m(3) (average 1.65 +/- 0.32 Bq/m(3), n=20) and increased toward the Antarctic Divergence, probably reflecting the influence of Ra-226-depleted meltwater and upwelled Ra-226-replete deep water. The fraction of meltwater, f(i) was estimated from 226Ra activity concentration and salinity using a three-component (along with Antarctic Summer Surface Water, and Prydz Bay Deep Water) mixing model. Although the fraction of meltwater is relatively minor (1.6-11.9%, average 4.1 +/- 2.7%, n=20) for the surface waters (sampled at similar to 6 m), a positive correlation between surface delta C-13(POC) and f(i) (delta C-13(POC)=0.94 x f(i) - 28.44, n=20, r(2)=0.66, p < 0.0001) was found, implying that sea ice melting may have contributed to elevated delta C-13(POC) values in the inner Prydz Bay compared to the open oceanic waters. This is the first time for a relationship between delta C-13(POC) and meltwater fraction to be reported in polar oceans to our knowledge. We propose that sea ice melting may have affected surface ocean delta C-13(POC) by enhancing water column stability and providing a more favorable light environment for phytoplankton photosynthesis, resulting in drawdown of seawater CO2 availability, likely reducing the magnitude of isotope fractionation during biological carbon fixation. Our results highlight the linkage of ice melting and delta C-13(POC), providing insights into understanding the carbon cycling in the highly productive Antarctic waters. (C) 2013 Elsevier Ltd. All rights reserved.